In reliable transmission protocols, what is the purpose of using sequence numbers on packets or frames?

Introduction / Context:
Reliable transmission protocols at the data link and transport layers, such as TCP or sliding window protocols, make extensive use of sequence numbers. These numbers are attached to packets or frames to help both sender and receiver manage the correct ordering of data and handle retransmissions when errors occur. Understanding the role of sequence numbers is fundamental for grasping how reliability, ordered delivery, and flow control are achieved in computer networks.

Given Data / Assumptions:

• The communication channel may lose, delay, or reorder packets.
• The protocol supports acknowledgements and retransmissions.
• Each packet or frame carries a sequence number field in its header.
• The receiver may buffer out of order packets until missing ones arrive.

Concept / Approach:
Sequence numbers serve several related purposes. First, they allow the receiver to determine the correct order of packets, even if the network delivers them in a different order. Second, they help detect lost packets because gaps in the sequence indicate missing data. Third, they allow detection of duplicate packets caused by retransmissions or delayed copies, ensuring that the receiver does not deliver the same data multiple times. In sliding window protocols, sequence numbers define the range of outstanding unacknowledged packets and help coordinate flow control between sender and receiver.

Step-by-Step Solution:
Step 1: Consider that the sender assigns a unique or cyclic sequence number to each outgoing packet or frame.Step 2: As packets travel through the network, some may be delayed, lost, or delivered out of order due to variable routes and congestion.Step 3: The receiver uses sequence numbers to place incoming packets into the correct order and to identify any gaps that indicate missing packets.Step 4: When a packet is detected as missing or when acknowledgements do not arrive, the sender can retransmit the specific sequence numbers that have not been confirmed.Step 5: If a duplicate packet arrives, the receiver recognises that its sequence number has already been processed and can discard the duplicate, preventing repeated delivery of data.

Verification / Alternative check:
To verify the role of sequence numbers, you can examine the behaviour of a protocol like TCP during a file transfer. Packet captures show each segment with an associated sequence number and acknowledgement number. When a segment is lost, the receiver sends acknowledgements indicating the last in order byte received. The sender then retransmits the missing sequence range. Because each segment has a unique sequence number range, the receiver can reassemble the file in the correct order and ignore any duplicate segments. This clearly demonstrates that sequence numbers provide both ordering and loss or duplication detection.

Why Other Options Are Wrong:
Option B claims that sequence numbers are used to encrypt payloads, which is incorrect; encryption is handled by separate security protocols such as TLS or IPsec. Option C suggests that sequence numbers reduce header size, but in reality they add fields to the header and do not directly increase payload capacity. Option D describes measuring physical distance by counting hops, which relates to routing metrics, not sequence numbering for reliability. These options do not capture the real purpose of sequence numbers in reliable transmission.

Common Pitfalls:
A common misunderstanding is to think that sequence numbers are only for ordering and not for loss or duplicate detection. In practice, they play all of these roles simultaneously. Another pitfall is ignoring the limited size of sequence number fields. When sequence numbers wrap around, protocols must handle this carefully to avoid confusion between old and new packets. Designers of reliable protocols must choose sequence number ranges and window sizes that prevent ambiguity, especially in high speed networks with large delays. Understanding these subtleties is important for advanced networking and protocol design work.

Final Answer:
Correct answer: To detect lost and duplicate packets and to ensure that data can be reordered correctly at the receiver